The invention relates generally to optical fiber sensing systems and methods, and more particularly relates to optical fiber sensing systems and methods for harsh environments.
Steady and transient temperature measurements are required in various industrial applications, including extremely harsh environments such as turbine engines, combustion cells, and power plants. Non-limiting examples of harsh environments include coal gasifiers and radiant syngas cooler vessels where the transient temperature typically ranges from 1000° F. to 3000° F. (537.7° C. to 1648.8° C.), with a pressure greater than 500 psi (3.45 MPa). Conventional sensors, such as thermocouples and pyrometers, are often difficult to use in such harsh environments due to package, penetration, and factors that may be present in these environments, such as, but not limited to, high temperatures, high pressures, presence of highly corrosive agents (H2S, SO2, H2O), and electromagnetic interference.
A fiber Bragg Grating (FBG) is a high quality reflector constructed in an optical fiber that reflects particular wavelengths of light and transmits other wavelengths. This is generally achieved by adding a periodic or quasiperiodic variation to a refractive index of the fiber core. The fibers could be sapphire, quartz or silica fiber materials. Sapphire fiber sensors typically have superior thermal survivability due to a melting point as high as −3700° F. (2037.7° C.), while tetrahedral fiber sensors typically survive up to ˜2100° F. (1148.8° C.). FBGs, either sapphire fiber-based Bragg grating sensor or tetrahedral fiber Bragg grating sensors, are highly desirable for multi-point temperature profile measurements due to their advantages in low mass, low specific heat, multiplexing, multi-point distribution, and electromagnetic interference immunity. Specifically, the multiplexing capability of these wavelength-encoded fiber sensors enables multi-point distributed sensing for thermal profile mapping with one fiber sensing cable, in which a plurality of Bragg grating elements are cascaded with a spatial resolution from a few millimeters to centimeters, and with wavelength resolution from 1 nanometer to a few nanometers.
However, the operation within a gasifier environment, characterized by high temperature, pressure, turbulence, and corrosion, not only can affect performance of the fiber sensors but also may shorten their service lifetime. Due to small diameters of 0.125 to 0.25 mm for typical fiber materials, high temperature FBG sensors have to be packaged before they are installed or embedded into a harsh environment structure. Potentially, the packaging of the sensor could protect the sensor from damage due to the hazardous environment and raise the survival rate of the sensor during the installation and service life. For packaged high temperature FBG sensors, durability and life span of the high temperature FBG sensors are not only dependent on the sensor itself but also on the packaging materials, packaging methods, and field installation.
Further, fiber cables are used to carry data from the sensors to the data acquisition and analysis instrumentation. In harsh environments, such instrumentation is typically positioned quite far away from the measurement environment. Transmission loss, either in the fiber sensors or the fiber cables, can result in the acquisition instrumentation receiving low quality sensor data, leading to inaccurate measurements. In currently known fibers, claddings are typically used to reduce transmission loss. But cladding materials typically delaminate at high temperatures because of differences in coefficient of thermal expansion (CTE) of the cladding materials with the fiber core.
Therefore, there is a need for providing a low-loss, field deployable, packaged fiber sensing cable and sensing system, for harsh environment applications.